Conventionally, defect detection on semiconductor wafers can be done with either optical or electron beam inspection. Systems and methods for inspecting semiconductor wafers for defects using optical and electron beam inspection techniques are well known.
Optical inspection systems frequently use either microscopic type imaging and/or the collection of the scattered energy. For the microscopic type of optical inspection, it may be difficult to inspect defects that generate little intensity change from the nominal structures. For example, dark defects on a dark background are typically difficult to detect due to the closeness of the change in intensity in the reflected image due to the dark defect on the dark background.
It has been found that, in some applications, defect detection can be improved by using phase detection rather than intensity based detection, because defects that create little intensity or little intensity change typically would have a modest phase signal.
One system for defect detection using phase detection is disclosed in U.S. Pat. No. 6,078,392, which is incorporated herein by reference in its entirety. This patent proposes a direct-to-digital holography approach wherein a collimated reference beam is incident upon a reference beam mirror at a non-normal angle, and the reference beam and an object beam that is focused at a focal plane of a digital recorder to form an image. This direct-to-digital holography approach, however, requires significant computational power, which may limit throughput. Further, this approach may be cumbersome by requiring the reference beam to be incident upon a reference beam mirror at a non-normal angle.
Another patent that refers to use of digital holograms is U.S. Pat. No. 6,262,818, the disclosure of which is incorporated herein by reference in its entirety. This patent refers to a method for simultaneous amplitude and quantitative phase contrast imaging by numerical reconstruction of digital holograms. This approach also requires significant computational power, which may limit throughput.